Erosion-corrosion resistant nitride cermets

ABSTRACT

The invention includes a cermet composition represented by the formula (PQ)(RS) comprising: a ceramic phase (PQ) and a binder phase (RS) wherein, P is a metal selected from the group consisting of Si, Mn, Fe, Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W and mixtures thereof, Q is nitride, R is a metal selected from the group consisting of Fe, Ni, Co, Mn and mixtures thereof, S consists essentially of at least one element selected from Cr, Al, Si, and Y, and at least one reactive wetting aliovalent element selected from the group consisting of Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W and mixtures thereof.

This application claims the benefit of U.S. Provisional application60/471,791 filed May 20, 2003.

FIELD OF INVENTION

The present invention is broadly concerned with cermets, particularlycermet compositions comprising a metal nitride. These cermets aresuitable for high temperature applications wherein materials withsuperior erosion and corrosion resistance are required.

BACKGROUND OF INVENTION

Erosion resistant materials find use in many applications whereinsurfaces are subject to eroding forces. For example, refinery processvessel walls and internals exposed to aggressive fluids containing hard,solid particles such as catalyst particles in various chemical andpetroleum environments are subject to both erosion and corrosion. Theprotection of these vessels and internals against erosion and corrosioninduced material degradation especially at high temperatures is atechnological challenge. Refractory liners are used currently forcomponents requiring protection against the most severe erosion andcorrosion such as the inside walls of internal cyclones used to separatesolid particles from fluid streams, for instance, the internal cyclonesin fluid catalytic cracking units (FCCU) for separating catalystparticles from the process fluid. The state-of-the-art in erosionresistant materials is chemically bonded castable alumina refractories.These castable alumina refractories are applied to the surfaces in needof protection and upon heat curing hardens and adheres to the surfacevia metal-anchors or metal-reinforcements. It also readily bonds toother refractory surfaces. The typical chemical composition of onecommercially available refractory is 80.0% Al₂O₃, 7.2% SiO₂, 1.0% Fe₂O₃,4.8% MgO/CaO, 4.5% P₂O₅ in wt %. The life span of the state-of-the-artrefractory liners is significantly limited by excessive mechanicalattrition of the liner from the high velocity solid particleimpingement, mechanical cracking and spallation. Therefore there is aneed for materials with superior erosion and corrosion resistanceproperties for high temperature applications. The cermet compositions ofthe instant invention satisfy this need.

Ceramic-metal composites are called cermets. Cermets of adequatechemical stability suitably designed for high hardness and fracturetoughness can provide an order of magnitude higher erosion resistanceover refractory materials known in the art. Cermets generally comprise aceramic phase and a binder phase and are commonly produced using powdermetallurgy techniques where metal and ceramic powders are mixed, pressedand sintered at high temperatures to form dense compacts.

The present invention includes new and improved cermet compositions.

The present invention also includes cermet compositions suitable for useat high temperatures.

Furthermore, the present invention includes an improved method forprotecting metal surfaces against erosion and corrosion under hightemperature conditions.

These and other objects will become apparent from the detaileddescription which follows.

SUMMARY OF INVENTION

The invention includes a cermet composition represented by the formula(PQ)(RS) comprising: a ceramic phase (PQ) and a binder phase (RS)wherein,

-   P is a metal selected from the group consisting of Si, Mn, Fe, Ti,    Zr, Hf, V, Nb, Ta, Cr, Mo, W and mixtures thereof,-   Q is nitride,-   R is a metal selected from the group consisting of Fe, Ni, Co, Mn    and mixtures thereof,-   S consists essentially of at least one element selected from Cr, Al,    Si, and Y, and at least one reactive wetting aliovalent element    selected from the group consisting of Ti, Zr, Hf, V, Nb, Ta, Cr, Mo,    W and mixtures thereof.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a scanning electron microscope (SEM) image of TiN cermet madeusing 30 vol % 304 stainless steel (SS) binder illustrating the TiNceramic phase particles dispersed in binder and reprecipitation of newphase M₂N where M is mainly Cr, Fe, and Ti.

FIG. 2 is a SEM image of CrN cermet made using 30 vol % 304SS binderillustrating CrN ceramic phase particles dispersed in binder and thereprecipitation of new phase M₂N where M is mainly Cr and Fe.

DETAILED DESCRIPTION OF THE INVENTION

One component of the cermet composition represented by the formula(PQ)(RS) is the ceramic phase denoted as (PQ). In the ceramic phase(PQ), P is a metal selected from the group consisting of Si, Mn, Fe, Ti,Zr, Hf, V, Nb, Ta, Cr, Mo, W and mixtures thereof. Thus the ceramicphase (PQ) in the nitride cermet composition is a metal nitride. Themolar ratio of P to Q in (PQ) can vary in the range of 1:3 to 3:1.Preferably in the range of 1:2 to 2:1. As non limiting illustrativeexamples, when P=Ti, (PQ) can be TiN wherein P:Q is about 1:1. When P=Crthen (PQ) can be Cr₂N wherein P:Q is 2:1. The ceramic phase impartshardness to the nitride cermet and erosion resistance at temperatures upto about 1000° C.

The ceramic phase (PQ) of the cermet is preferably dispersed in thebinder phase (RS). It is preferred that the size of the dispersedceramic particles is in the range 0.5 to 3000 microns in diameter. Morepreferably in the range 0.5 to 100 microns in diameter. The dispersedceramic particles can be any shape. Some non-limiting examples includespherical, ellipsoidal, polyhedral, distorted spherical, distortedellipsoidal and distorted polyhedral shaped. By particle size diameteris meant the measure of longest axis of the 3-D shaped particle.Microscopy methods such as optical microscopy (OM), scanning electronmicroscopy (SEM) and transmission electron microscopy (TEM) can be usedto determine the particle sizes. In another embodiment of thisinvention, the ceramic phase (PQ) is dispersed as platelets with a givenaspect ratio, i.e., the ratio of length to thickness of the platelet.The ratio of length:thickness can vary in the range of 5:1 to 20:1.Platelet microstructure imparts superior mechanical properties throughefficient transfer of load from the binder phase (RS) to the ceramicphase (PQ) during erosion processes.

Another component of the nitride cermet composition represented by theformula (PQ)(RS) is the binder phase denoted as (RS). In the binderphase (RS), R is the base metal selected from the group consisting ofFe, Ni, Co, Mn and mixtures thereof. S is an alloying metal consistingessentially of at least one element selected from Cr, Al, Si, and Y,and, at least one reactive wetting aliovalent element selected form thegroup consisting of Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W and mixturesthereof. The combined weight of Cr, Al, Si, Y and mixtures thereof areat least about 12 wt % based on the weight of the binder (RS). Thereactive wetting aliovalent element is about 0.01 wt % to about 5 wt %,preferably about 0.01 wt % to about 2 wt % of based on the weight of thebinder. The elements Ti, Zr, Hf, Ta provide enhanced wetting by reducingthe contact angle between the ceramic (PQ) and binder phases (RS) in thetemperature range of 1300° C. to 1750° C. These elements can be added asa pure element during mixing of the nitride and metal powder inprocessing or can be part of the metal powder prior to mixing withnitride powder. The elements Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W arealiovalent elements characterized by multivalent states when in anoxidized state. These elements decrease defect transport in the oxidescale thereby providing enhanced corrosion resistance.

In the nitride cermet composition the binder phase (RS) is in the rangeof 5 to 70 vol %, preferably 5 to 45 vol %, and more preferably 5 to 30vol %, based on the volume of the cermet. The mass ratio of R to S canvary in the range from 50/50 to 90/10. In one preferred embodiment thechromium content in the binder phase (RS) is at least 12 wt % based onthe weight of the binder (RS). In another preferred embodiment thecombined zirconium and hafnium content in the binder phase (RS) is about0.01 wt % to about 2.0 wt % based on the total weight of the binderphase (RS).

The cermet composition can further comprise secondary nitrides (P′Q)wherein P′ is selected from the group consisting of Si, Mn, Fe, Ti, Zr,Hf, V, Nb, Ta, Cr, Mo, W, Ni, Co, Al, Y, and mixtures thereof. Stateddifferently, the secondary nitrides are derived from the metal elementsfrom P, R, S and combinations thereof of the cermet composition(PQ)(RS). The ratio of P′ to Q in (P′Q) can vary in the range of 1:3 to3:1. The total ceramic phase volume in the cermet of the instantinvention includes both (PQ) and the secondary nitrides (P′Q). In thenitride cermet composition (PQ)+(P′Q) ranges from of about 30 to 95 vol% based on the volume of the cermet. Preferably from about 55 to 95 vol% based on the volume of the cermet. More preferably from 70 to 90 vol %based on the volume of the cermet.

The volume percent of cermet phase (and cermet components) excludes porevolume due to porosity. The cermet can be characterized by a porosity inthe range of 0.1 to 15 vol %. Preferably, the volume of porosity is 0.1to less than 10% of the volume of the cermet. The pores comprising theporosity is preferably not connected but distributed in the cermet bodyas discrete pores. The mean pore size is preferably the same or lessthan the mean particle size of the ceramic phase (PQ).

One aspect of the invention is the micro-morphology of the cermet. Theceramic phase can be dispersed as spherical, ellipsoidal, polyhedral,distorted spherical, distorted ellipsoidal and distorted polyhedralshaped particles or platelets. Preferably, at least 50% of the dispersedparticles is such that the particle-particle spacing between theindividual nitride ceramic particles is at least about 1 nm. Theparticle-particle spacing may be determined for example by microscopymethods such as SEM and TEM.

The cermet compositions of the instant invention possess enhancederosion and corrosion properties. The erosion rates were determined bythe Hot Erosion and Attrition Test (HEAT) as described in the examplessection of the disclosure. The erosion rate of the nitride cermets ofthe instant invention is less than 1.0×10⁻⁶ cc/gram of SiC erodant. Thecorrosion rates were determined by thermogravimetric (TGA) analyses asdescribed in the examples section of the disclosure. The corrosion rateof the nitride cermets of the instant invention is less than 1×10⁻¹⁰gm²/cm⁴ sec.

The cermets of the instant invention possess fracture toughness ofgreater than about 3 MPa·m^(1/2), preferably greater than about 5MPa·m^(1/2), and more preferably greater than about 10 MPa·m^(1/2).Fracture toughness is the ability to resist crack propagation in amaterial under monotonic loading conditions. Fracture toughness isdefined as the critical stress intensity factor at which a crackpropagates in an unstable manner in the material. Loading in three-pointbend geometry with the pre-crack in the tension side of the bend sampleis preferably used to measure the fracture toughness with fracturemechanics theory. (RS) phase of the cermet of the instant invention asdescribed in the earlier paragraphs is primarily responsible forimparting this attribute.

Another aspect of the invention is the avoidance of embrittlingintermetallic precipitates such as sigma phase known to one of ordinaryskill in the art of metallurgy. The nitride cermet of the instantinvention has preferably less than about 5 vol % of such embrittlingphases. The cermet of the instant invention with (PQ) and (RS) phases asdescribed in the earlier paragraphs is responsible for imparting thisattribute.

The cermet compositions are made by general powder metallurgicaltechnique such as mixing, milling, pressing, sintering and cooling,employing as starting materials a suitable ceramic powder and a binderpowder in the required volume ratio. These powders are milled in a ballmill in the presence of an organic liquid such as ethanol for a timesufficient to substantially disperse the powders in each other. Theliquid is removed and the milled powder is dried, placed in a die andpressed into a green body. The resulting green body is then sintered attemperatures above about 1200° C. up to about 1750° C. for times rangingfrom about 10 minutes to about 4 hours. The sintering operation ispreferably performed in an inert atmosphere or a reducing atmosphere orunder vacuum. For example, the inert atmosphere can be argon and thereducing atmosphere can be hydrogen. Thereafter the sintered body isallowed to cool, typically to ambient conditions. The cermet preparedaccording to the process of the invention allows fabrication of thecermet exceeding 5 mm in thickness.

One feature of the cermets of the invention is their microstructuralstability, even at elevated temperatures, making them particularlysuitable for use in protecting metal surfaces against erosion attemperatures in the range of up to about 1000° C. It is believed thisstability permits their use for time periods greater than 2 years, forexample for about 2 years to about 10 years. In contrast many knowncermets undergo transformations at elevated temperatures which resultsin the formation of phases which have a deleterious effect on theproperties of the cerrnet.

The high temperature stability of the cermets of the invention makesthem suitable for applications where refractories are currentlyemployed. A nonlimiting list of suitable uses include liners for processvessels, transfer lines, cyclones, for example, fluid-solids separationcyclones as in the cyclone of Fluid Catalytic Cracking Unit used inrefining industry, grid inserts, thermo wells, valve bodies, slide valvegates and guides, catalyst regenerators, and the like. Thus, metalsurfaces exposed to erosive or corrosive environments, especially atabout 300° C. to about 1000° C. are protected by providing the surfacewith a layer of the cermet compositions of the invention. The cermets ofthe instant invention can be affixed to metal surfaces by mechanicalmeans or by welding.

EXAMPLES

Determination of Volume Percent:

The volume percent of each phase, component and the pore volume (orporosity) were determined from the 2-dimensional area fractions by theScanning Electron Microscopy method. Scanning Electron Microscopy (SEM)was conducted on the sintered cermet samples to obtain a secondaryelectron image preferably at 1000× magnification. For the area scannedby SEM, X-ray dot image was obtained using Energy Dispersive X-raySpectroscopy (EDXS). The SEM and EDXS analyses were conducted on fiveadjacent areas of the sample. The 2-dimensional area fractions of eachphase was then determined using the image analysis software: EDXImaging/Mapping Version 3.2 (EDAX Inc, Mahwah, N.J. 07430, USA) for eacharea. The arithmetic average of the area fraction was determined fromthe five measurements. The volume percent (vol %) is then determined bymultiplying the average area fraction by 100. The vol % expressed in theexamples have an accuracy of +/−50% for phase amounts measured to beless than 2 vol % and have an accuracy of +/−20% for phase amountsmeasured to be 2 vol % or greater.

Determination of Weight Percent:

The weight percent of elements in the cermet phases was determined bystandard EDXS analyses.

The following non-limiting examples are included to further illustratethe invention.

Example 1

70 vol % of 2-5 μm average diameter of TiN powder (99.8% purity, fromAlfa Aesar) and 30 vol % of 6.7 μm average diameter 304SS powder (OspreyMetals, 95.9% screened below −16 μm) were dispersed with ethanol in HDPEmilling jar. The powders in ethanol were mixed for 24 hours with YttriaToughened Zirconia (YTZ) balls (10 mm diameter, from Tosoh Ceramics) ina ball mill at 100 rpm. The ethanol was removed from the mixed powdersby heating at 130° C. for 24 hours in a vacuum oven. The dried powderwas compacted in a 40 mm diameter die in a hydraulic uniaxial press(SPEX 3630 Automated X-press) at 5,000 psi. The resulting green discpellet was ramped up to 400° C. at 25° C./min in argon and held at 400°C. for 30 min for residual solvent removal. The disc was then heated to1500° C. and held at 1500° C. for 2 hours at 15° C./min in argon. Thetemperature was then reduced to below 100° C. at −15° C./min.

The resultant cermet comprised:

-   i) 70 vol % TiN with average grain size of about 4 μm-   ii) 2 vol % secondary nitride M₂N with average grain size of about 1    μm, where M=68Cr:20Fe:12Ti in wt %-   iii) 28 vol % Cr-depleted alloy binder (71Fe:11Ni:15Cr:3Ti in wt %).

FIG. 1 is a SEM image of TiN cermet processed according to this example,wherein the bar represents 5 μm. In this image the TiN phase appearsdark and the binder phase appears light. The Cr-rich secondary M₂N phaseis also shown in the binder phase. By Cr-rich is meant that the metal Cris of higher proportion than the other constituent metals (M) of thesecondary nitride M₂N.

Example 2

70 vol % of CrN powder (99.8% purity, from Alfa Aesar, 99% screenedbelow 325 mesh) and 30 vol % of 6.7 μm average diameter 304SS powder(Osprey Metals, 95.9% screened below −16 μm) were used to process thecermet disc as described in Example 1. The cermet disc was then heatedto 1450° C. and held at 1450° C. for 1 hour at 15° C./min in argon. Thetemperature was then reduced to below 100° C. at −15° C./min.

The resultant cermet comprised:

-   i) 20 vol % CrN with average grain size of about 25 μm-   ii) 50 vol % secondary nitride M₂N with average grain size of about    1 μm, where M=Cr, Fe, Ni-   iii) 30 vol % Cr-depleted alloy binder.

FIG. 2 is a SEM image of CrN cermet processed according to this example,wherein the bar represents 50 μm. In this image the CrN phase appearsdark and the binder phase appears light. The Cr-rich secondary M₂N phaseis also shown in the binder phase.

Example 3

Each of the cermets of Examples 1 and 2 was subjected to a hot erosionand attrition test (HEAT). The procedure employed was as follows:

1) A specimen cermet disk of about 35 mm diameter and about 5 mm thickwas weighed.

2) The center of one side of the disk was then subjected to 1200 g/minof SiC particles (220 grit, #1 Grade Black Silicon Carbide, UKabrasives, Northbrook, Ill.) entrained in heated air exiting from a tubewith a 0.5 inch diameter ending at 1 inch from the target at an angle of45°. The velocity of the SiC was 45.7 m/sec.

3) Step (2) was conducted for 7 hours at 732° C.

4) After 7 hours the specimen was allowed to cool to ambient temperatureand weighed to determine the weight loss.

5) The erosion of a specimen of a commercially available castablerefractory was determined and used as a Reference Standard. TheReference Standard erosion was given a value of 1 and the results forthe cermet specimens are compared in Table 1 to the Reference Standard.In Table 1 any value greater than 1 represents an improvement over theReference Standard. TABLE 1 Starting Finish Weight Bulk ImprovementCermet Weight Weight Loss Density Erodant Erosion [(Normalized {Example}(g) (g) (g) (g/cc) (g) (cc/g) erosion)⁻¹] TiN/304SS {1} 17.9379 15.87242.0655 6.200 5.04E+5 6.6100E−7 1.6 CrN/304SS {2} 19.8637 17.7033 2.16046.520 5.04E+5 4.9576E−7 2.1

Example 4

Each of the cermets of Examples 1 and 2 was subjected to an oxidationtest. The procedure employed was as follows:

1) A specimen cermet of about 10 mm square and about 1 mm thick waspolished to 600 grit diamond finish and cleaned in acetone.

2) The specimen was then exposed to 100 cc/min air at 800° C. inthermogravimetric analyzer (TGA).

3) Step (2) was conducted for 65 hours at 800° C.

4) After 65 hours the specimen was allowed to cool to ambienttemperature.

5) Thickness of oxide scale was determined by cross sectional microscopyexamination of the corrosion surface.

6) In Table 2 any value less than 150 μm represents acceptable corrosionresistance. TABLE 2 Cermet {Example} Thickness of Oxide Scale (μm)TiN-30 304SS {1} 110.0 CrN-25 304SS {2} 1.5

1-14. (canceled)
 15. A method for protecting a metal surface subject toerosion at temperatures up to 1000° C., the method comprising providingthe metal surface with a cermet composition represented by the formula(PQ)(RS) comprising: a ceramic phase (PQ) and a binder phase (RS)wherein, P is a metal selected from the group consisting of Si, Mn, Fe,Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W and mixtures thereof, Q is nitride, Ris a metal selected from the group consisting of Fe, Ni, Co, Mn andmixtures thereof, S consists essentially of at least one elementselected from Cr, Si, Y and mixtures thereof, and at least one reactivewetting aliovalent element selected from the group consisting of Ti, Zr,Hf, V, Nb, Ta, Cr, Mo, W and mixtures thereof, wherein the combinedweights of said Cr, Si, and Y and mixtures thereof is at least 12 wt %based on the weight of the binder phase (RS) and wherein the ceramicphase (PQ) ranges from about 30 to 95 vol % based on the volume of thecermet.
 16. The method of claim 15 wherein said surface is subjected toerosion at temperatures in the range of 300° C. to 1000° C.
 17. Themethod of claim 15 wherein said surface comprises the inner surface of afluid-solids separation cyclone. 18-30. (canceled)
 31. The method ofclaim 15 wherein the molar ratio of P:Q in the ceramic phase (PQ) canvary in the range of 1:3 to 3:1.
 32. The method of claim 15 wherein (PQ)ranges from of about 55 to 95 vol % based on the volume of the cermet.33. The method of claim 15 wherein said ceramic phase (PQ) is dispersedin the binder phase (RS) as spherical particles in the size range of 0.5microns to 3000 microns diameter.
 34. The method of claim 15 wherein thebinder phase (RS) is in the range of 5 to 70 vol % based on the volumeof the cermet and the mass ratio of R to S ranges from 50/50 to 90/10.35. The method of claim 15 wherein said at least one reactive wettingaliovalent element selected from the group consisting of Ti, Zr, Hf, V,Nb, Ta, Cr, Mo, W and mixtures thereof is in the range of 0.01 to 5 wt %based on the total weight of the binder phase (RS).
 36. The method ofclaim 15 further comprising a secondary nitride (P′Q) wherein P′ isselected from the group consisting of Si, Mn, Fe, Ti, Zr, Hf, V, Nb, Ta,Cr, Mo, W, Ni, Co, Al, Y, and mixtures thereof.
 37. The method of claim15 having a fracture toughness of greater than about 3 MPa m^(1/2). 38.The method of claim 15 having an erosion rate less than about 1×10⁻⁶cc/gram loss when subject to 1200 g/min of 10 μm to 100 μm SiC particlesin air with an impact velocity of at least about 45.7 m/sec (150 ft/sec)and at an impact angle of about 45 degrees and a temperature of at leastabout 732° C. (1350° F.) for at least 7 hours.
 39. The method of claim15 having corrosion rate less than about 1×10⁻¹⁰ g²/cm⁴·s or an averageoxide scale of less than 150 μm thickness when subject to 100 cc/min airat 800° C. for at least 65 hours.
 40. The method of claim 15 having anerosion rate less than about 1×10⁻⁶ cc/gram when subject to 1200 g/minof 10 μm to 100 μm SiC particles in air with an impact velocity of atleast about 45.7 m/sec (150 ft/sec) and at an impact angle of about 45degrees and a temperature of at least about 732° C. (1350° F.) for atleast 7 hours and a corrosion rate less than about 1×10⁻¹⁰ g²/cm⁴·s oran average oxide scale of less than 150 μm thickness when subject to 100cc/min air at 800° C. for at least 65 hours.
 41. The method of claim 15having embrittling phases less than about 5 vol % based on the volume ofthe cermet.
 42. The method of claim 15 wherein the overall thickness ofthe cermet composition is greater than 5 millimeters.